Today's industrial furnace castings are almost invariably cast from alloys selected from the standard alloys specified by the Alloy Castings Institute division of the Steel Founders Society of America (SFSA-ACI) or from various modifications of these alloys. These standard alloys generally contain 19% to 32% chromium, 9% to 68% nickel, 0.20% to 0.75% carbon, with the balance being substantially iron (all elemental percentages herein are in terms of weight percent unless otherwise specified). These alloys also often contain small amounts of manganese and silicon, as employed in steel-making practice, and even smaller amounts of impurities. The exact proportions of these elements vary from grade to grade. The grade designated as type HP has the best hot strength of all the standard ACI grades and nominally contains about 35% nickel, 25% chromium and 0.45% carbon.
Over a period of several decades, efforts have been made to improve the properties of these alloys by increasing the chromium, nickel, manganese and/or silicon content as well as adding one or more other selected elements.
In the case of precision castings for use in high temperature applications where the casting must also be resistant to abrasion, corrosion, impact, and thermal shock and must maintain dimensional stability, very difficult and expensive production methods and expensive alloys have been employed to address these requirements. Typical of such applications are castings for jet engine turbine rotor blades. In the case of alloys for rotor blade castings, which are usually nickel-base or cobalt-base alloys, the alloys may contain scarce elements in various proportions such as up to several percent by weight of two or more elements from the tantalum or cobalt pair, the quite scarce vanadium, columbium (niobium) and zirconium group, the fairly scarce tungsten and titanium pair, and/or more plentiful elements from the molybdenum and boron pair. Virtually every other element that is metallurgically compatible has also been tried in various combinations and proportions for this application.
Because of the great differences in tonnage of furnace part castings versus the tonnage in rotor blade castings the truly scarce elements, such as tantalum and columbium, have generally not been used in castings for industrial furnace parts. Furthermore, the use of the less scarce but still expensive alloying elements in furnace part castings can only be justified if those elements are effective as small fractions of alloy compositions. In particular, it is very desirable to reduce the nickel content of alloys for furnace part castings whenever possible, nickel being relatively expensive and constituting a major proportion of such alloys.
An additional consideration in the development of alloys for furnace castings is the now well established fact that for high hot strength and reasonably long service life at furnace temperatures of about 1500.degree. F. to about 2000.degree.-2100.degree. F. such alloys must retain wholly austenitic (face-centered cubic) matrix structures. Ferritic (body-centered cubic) matrix or unstable matrix structures must be avoided.
Roy et al., U.S. Pat. No. 3,165,400, discloses alloys said to have an austenitic structure at room temperature useful for turbine rotor blade castings as well as other applications where temperatures up to about 1500.degree. are encountered. Roy et al. broadly disclose a variety of alloy compositions described (col. 5, line 36 to col. 6, line 35) as having about 0.8 to 1.25% carbon, about 2 to 8% nickel, about 1 to 15% manganese, about 12 to 35% chromium, and a plurality of elements selected from molybdenum, tungsten and metals from the group columbium and tantalum in amounts not greater than about 12%. Various provisos are disclosed with respect to the maximum amounts of certain combinations of these elements. Additional desirable elements are said to include 0 to about 2.5% silicon, 0 to about 0.6% nitrogen, and 0 to about 8% cobalt. Also disclosed is the replacement of a portion of the iron in the alloys with up to 5% titanium, up to 5% vanadium, up to 1% boron, and up to 0.2% phosphorus, plus incidental elements such as zirconium, aluminum and magnesium. Nevertheless, the alloys of Roy et al. must contain at least 40% iron.
Even though the alloys of Roy et al. are said to have an austenitic structure at room temperature, the alloys of Roy et al. would not be expected to remain austenitic in service at high temperatures, for even such short periods as several hours up to about two months, based upon the test results and properties set forth in the patent. For example, while the photomicrographs of the Roy et al. alloys (FIGS. 1-7) show the presence of austenite, carbides are also present after only 100 hours (4 days) of exposure at 1500.degree. F. Accordingly, based upon their compositions, the exemplary alloys of Roy et al. are almost all prone to the detrimental formation of sigma phase at service temperatures, drastically limiting service life. Thus, while such alloys may remain free of sigma phase for a limited time, critical amounts would be expected to form after two or three months of service have passed.
At about the time the Roy et al. patent issued (1965) many metallurgists hoped and believed that substantial amounts of manganese, possibly along with quantities of nitrogen, could be substituted for all or most of the nickel in heat-resistant alloys to provide alloys having improved heat resistance. These hopes met with failure. Low-nickel, high-manganese, heat-resistant alloys that gave good results in the 100 or 1000 hour creep and rupture tests were prone to embrittlement and failure in service. Thus, while Roy et al. disclose very broad component ranges, they do not teach how to formulate alloys of sufficiently high chromium and low nickel contents to withstand hot gas corrosion above about 1650.degree. F. Further, the reference does not teach how to formulate low-nickel alloys that retain a truly stable austenitic structure at high temperatures for the service life periods typically required of furnace parts.